1. Field of the Invention
This invention relates to a flame retardant polycarbonate resin composition having improved optical transparency.
2. Prior Art
Flame retardant resin compositions are often used in various products such as electric and electronic parts, building members, automotive parts, and daily goods. These resin compositions are generally made flame retardant by adding organic halogen compounds optionally in admixture with antimony trioxide. However, these flame retardant resin compositions have the drawback that harmful halogen gases evolve upon combustion.
It is also known that resin compositions can be made flame retardant by adding silicone resins which do not evolve harmful gases.
JP-B 60421/1987 discloses a flame retardant resin composition comprising an organic resin and a silicone resin containing at least 80% by weight of trifunctional siloxane units. In consideration of the melt processing with the organic resin, a silicone resin consisting essentially of trifunctional siloxane units is used. Since such a silicone resin imparts less flame retardance, more than 10% by weight of the silicone resin must be added in order to achieve a satisfactory flame retardant effect.
JP-B 31513/1988 discloses a thermal oxidation resistant resin composition in which an alkoxy-terminated silicone resin is added. A liquid low molecular weight silicone having a high alkoxy group content is used. The silicone of this type, even when added in a minor amount, can have substantial influence on the outer appearance and strength of molded resin parts and tends to bleed out of molded resin parts. The silicone of this type is susceptible to hydrolysis to form as by-products flammable low-melting compounds such as alcohols. Then, no satisfactory flame retardant effect is expected.
JP-B 48947/1991, 78171/1996, and 33971/1996 disclose flame retardant resin compositions in which silicone resins consisting of monofunctional and tetrafunctional siloxane units are added. JP-A 128434/1995 discloses a flame retardant resin composition in which a silicone resin containing vinyl-bearing siloxane units is added. However, in order for these compositions to exert satisfactory flame retardant effects, the amount of silicone resin must be increased, and inorganic fillers such as aluminum hydroxide must be used in admixture with halogen or phosphorus compounds.
As discussed above, the addition of silicone resins arises the problem that no satisfactory flame retardant effect is obtained unless the amount of silicone resin added is increased, whereas an increased amount of silicone resin can drastically exacerbate the moldability of resin compositions and the outer appearance and mechanical strength of molded resin parts. Efforts were made to develop silicone resin additives having more flame retardant effect or additives capable of cooperating with silicone resins to improve their flame retardant effect.
JP-A 176425/1996 discloses a flame retardant resin composition comprising a polycarbonate resin, an epoxybearing organopolysiloxane, and an alkali metal salt of an organic sulfonic acid. JP-A 176427/1996 discloses a flame retardant resin composition comprising a polycarbonate resin, a polycarbonate resin modified with a phenolic hydroxyl-bearing organopolysiloxane, and an organic alkali metal salt. Further, JP-A 169914/1997 discloses a composition wherein a petroleum heavy oil or pitch is combined with a silicone compound for improving flame retardance. These silicone resins having special organic functional groups are expensive because of the complication of their preparation process, but do not achieve a sufficient flame retardant effect to compensate for the increased cost. Also, the heavy oil and pitch have disadvantage on moldability of the plastic containing them. In this regard, a further improvement is desired.
The above-mentioned prior art silicone resins for imparting flame retardance are less dispersible in or less compatible with polycarbonate resins. As a result, there are obtained molded parts of the island-in-sea structure in which two components having different indexes of refraction are present as separate phases. These molded parts have a low transparency and are even opaque when large amounts of the silicone resins are blended.
It is thus desired to have a technique of imparting flame retardance to polycarbonate resins while maintaining the physical properties and optical transparency thereof.